The playback of voicemail messages on your computer requires the IP address of the voicemail server to be entered into your browsers list of proxy server exceptions.

Google Chrome
Click on the customize icon and select Options. Select Under the Hood and click on Change proxy settings. Select Connections | LAN settings | Advanced. Enter the IP address of the voicemail server in the Exceptions list.

Firefox
Select Tools | Options | Network | Setting | Manual proxy configuration. Enter the IP address of the voicemail server in the No proxy for list.